Along with the idiosyncracies of each animal's behavior, they've animated the communication methods of each animal. Including the human animal. For example, when Toothless wants to show he's cranky at Hiccup, Toothless turns around and presents his back parts. That's something that both a rabbit and a horse will do to communicate that you're not worth his time. Toothless communicates to Stormfly--and Valka--in bird language, with the head-bobbing.

Although Toothless already seemed fluent in Dog, it seemed to me that Toothless was learning more Dog at Valka's sanctuary--he seemed quite surprised at the butt-sniffing thing. Which makes sense as pack behavior seems most obvious in canine societies. At least most obvious to humans.

Toothless is now Alpha. Now Toothless can use all those languages he's learned to function as the Alpha. Maybe it will be a Doctor Doolittle thing (the books, not the movies.) Or a Tarzan thing (The movie, not the books). I remember a scene where Tarzan is talking with the baby baboon about Jane's sketch--that sort of scene, rather than chumming around with his pals in Human. So the animators will be able to riff along those lines in the next movie, now that Toothless is Alpha.
Also since the Bewilderbeasts were displaying primate behavior (hey, humans display primate behavior--what a concept) I wouldn't be surprised if Toothless took the cue and started behaving in primate, or even elephant, to communicate Alpha.
But I would like to se the return of the cat dialect, if nothing else than because cat was originally so prevalent in Toothless's genome.

Toothless is now back in sync with Hiccup, as of the end of the movie. In fact, if they hadn't taken Toothless out of sync with Hiccup during the movie, I don't think Toothless being hypnotized by the Bewilderbeast would have worked. Hiccup didn't seem to have any problem standing up to the Good Alpha, while Toothless felt he needed to bow. I was a bit surprised by that, actually. But for this plot to work, that needed to be shown. Cats do pack up in prides, but I don't think that the non-alpha cats look as submissive as that to the alpha cats.

But the reason they should add cat here is because it puts Toothless back in sync with Hiccup the inventor--cats will learn how to operate doorknobs and other devices, learn the behaviors of mice and birds, and their behavior while doing this is not Dog, Rabbit, or even Primate. Also Toothless needs to get back in sync with Hiccup the Disobedient, someone with his own mind. And that comes out most loudly in Cat.

My opinion, and just something where I see possibility. Not intended to tell the animators "Thou Shalt Do Thusly" because that might not serve the story that they actually plan to tell.

Source1: http://movies.about.com/od/howtotrainyo ... trivia.htm

Source 2: http://open.salon.com/blog/amber_nasrul ... y_a_red_ta

Ok for those of you saying "Toothless is Toothless" and "He's not any animal" think about this. The animators, to make Toothless' distinct personality, combined MANY different animals. They included all sorts of animal sounds and behavior to make him seem like something out of this world. That's what makes him Toothless, but remember he does display different animal traits. It's how he was invented. It's not being disrespectful of Toothless because who Toothless is IS actually many animals combined. They used elephant, horse, tiger, etc sounds for him and even some humans for some sounds. That was just for the first movie, google "Toothless actor" and you'll come up with how they made Toothless' voice. Google it, you'll find that they did base his personality off of cats and dogs, as well as many other creatures. Each dragon is unique because each one obviously has a different combination of animal traits. Martian is right in the spoiler. Remember just because Toothless is adorable and one of a kind doesn't make how he was invented (with sounds, animal behavior specialists weighing in, on pen and paper and on a computer) mean him any disrespect. If anything, you should praise DreamWorks for being genius enough to make each dragon personality so unique by incorporating so many traits, sounds, animals, etc.

Horse + Wombat + Kangaroo + Bat + Snake + Cat + Dog + Panther + Bird of Prey

Ostrich + Tyrannosaurus Rex + Parrot

Pit Bull + Crocodile + Harley Davidson + Helicopter

Rhino + Truffle Pig + Dung Beetle + Jackhammer + Battle Axe

Cobra + Horned Lizard + Komodo Dragon

Owl + Great Dane + Bat + X-Wing Starfighter

If the story requires Hiccup and Toothless to grow apart, then perhaps a reversion to feline mannerisms would work well. However, if the story requires them to sacrifice their relationship when events come to a head, the most potent build-up would involve reminding us just how close together Hiccup and Toothless have grown.